The present invention relates to intrusion detection devices, and particularly to a method and apparatus for use in installing an infrared sensor as part of an intrusion detection system in such a way that it will not be affected adversely by the presence of heat sources such as lamps, windows, and heating system outlets and radiators.
Passive infrared sensors incorporating film, crystal, or ceramic pyroelectric detectors as sensitive elements are well known for use in detecting intruders in protected spaces. The body heat of a person moving through the zone of coverage of such an infrared sensor is sufficient for detection. However, any surface or object which can change temperature rather quickly, such as an incandescent lamp, a hot air register, furnace radiator, or exposed window can also be the source of sufficient infrared radiation to be detected by such a sensor. Such infrared radiation can trigger an alarm response unless provision has been made for preventing infrared radiation from such known sources from reaching the sensitive element of the infrared sensor.
In the past it has been difficult and time-consuming to determine clearly whether the field of coverage of any individual infrared sensor will be adequate to protect a space in which the sensor is to be located. Similarly, it has previously been difficult to determine except by trial and error testing whether incidental heat sources within a space to be protected by an infrared sensor are likely to cause problems. In the past installation of intrusion detection system infrared sensors has therefore been largely by trial and error installation of each sensor, with no way to preview accurately what potential sources of infrared radiation of no interest are located where they might be sensed by the intrusion detector system's passive infrared sensors. An experienced installer would place an infrared sensor in a location where good results were expected, but a "walk-through" test would then have to be performed to discover the actual location of the areas or beams of sensitivity of the infrared sensor, and thus to determine whether the coverage of the sensor or combination of sensors was satisfactory to detect the presence of an intruder in the space being protected.
As an improvement on such trial and error methods of installation, Mudge U.S. Pat. No. 4,275,303 teaches the use of a lamp to shine beams of visible light back through the lens of an infrared sensor. The sensor can be moved until the beam of light is visible to a person located in a zone where coverage is desired.
Carlson U.S. Pat. No. 4,642,454 teaches the use of a mirror in conjunction with an infrared sensor to view the fields of coverage of an infrared sensor through the lens of the sensor. Many infrared lenses, however, are opaque to visible light, and the Carlson invention is thus useless for infrared sensors including such lenses.
Cohen et al. U.S. Pat. No. 3,924,120 discloses an infrared detector utilizing a memory system to detect changes in the infrared radiation within a field covered by the detector. Such a system, however, is relatively complex and could not easily be utilized in the process of installing infrared sensors of the type commonly used in intrusion detection alarm systems.
Pistor U.S. Pat. No. 4,760,267 discloses a way of providing a black and white photograph of the pattern of infrared radiation received by an infrared sensor. The Pistor teachings, however, do not seem to be applicable to use during installation of an infrared sensor, in part because of the amount of time required.
Bechet et al. U.S. Pat. No. 4,773,752 discloses transmission of visible light to a television camera associated with an infrared camera utilized in a motion-stablizied infrared-detecting sighting device useful in controlling weapons. It is not clear how such a system could be used in installation of infrared sensors in intrusion detection systems.
Macall U.S. Pat. No. 4,081,678 discloses a system for viewing visible light along the same objective axis as an infrared optical system utilized as a temperature detecting device, but does not disclose how the system could be utilized in connection with intrusion detection systems.
Scofield U.S. Pat. No. 4,709,153, Keller-Steinbach U.S. Pat. No. 4,523,095, Stauffer U.S. Pat. No. 4,317,992 and Ariessohn et al. U.S. Pat. No. 4,539,588 also refer to infrared radiation sensors, but are not directly related to the problem of proper and effective installation of infrared sensors as part of intrusion detection systems.
What is still needed, then, is a method and apparatus for quickly, reliably, and simply determining whether a proposed location and orientation are appropriate for mounting of a passive infrared sensor as a part of an intrusion detection system, or whether such a proposed location or orientation result in reception of infrared radiation which would interfere with effective operation of such a sensor. Additionally, an easy and effective method is desired for making such an infrared sensor insensitive to known sources of infrared radiation which cannot be avoided practically by mounting the infrared sensor in a different location.